Diarrheal diseases are a major killer of children under the age of 5 years (U5) around the worid. Despite this long-recognized association, the precise causes of severe disease remain enigmatic. In an ongoing multisite study of diarrhea in U5 children, the PI will generate a priceless archive of bacterial isolates, fecal nucleic acids and frozen stools themselves from up to 14,000 cases and matched controls. We will leverage this unparalleled set of specimens, in the context of outstanding laboratory investigations into enteric pathogenesis, to address longstanding and critically important research questions. This work will be organized in 3 specific aims. Aim 1. Host genomic biomarkers associated with severe outcome in enteric disease. We will leverage our extensive database and specimen collection to determine the importance of single nucleotide polymorphisms (SNPs) previously associated with diarrhea, including IL-8, IL-10, lactofen'in, and osteoprotegerin genes. We will score these SNPs in host DNA from stool samples and correlate these findings with presence of severe diarrhea and death in the study database. Aim 2. Bacterial interaction in Diarrheal Disease. Most pathogenesis research examines the behavior of a single organism in pure culture, yet the real worid comprises innumerable interactions among living organisms. Our extensive laboratory expertise in pathogenesis allows us to formulate hypothesis-driven questions about the roles of inter-bacterial interaction in the study subjects. Aim 3. Pathogenomics of Shigella species. We will work with the University of Maryland Institute for Genome Sciences to perform the most comprehensive genomic study of Shigella strains ever performed, leveraging the availability of clinically characterized strains. Preliminary studies have revealed certain previously unsuspected genes that are more common among Shigella isolates from Kenyan patients who died of their disease. The association of these genes with severe diarrhea will be tested. This study will advance our knowledge of the most diarrhea cases in U5 subjects.